SYSV: Implement downloading and installing SYSV packages from HTTP

pkgadd has different syntax for different sources (datastream and file
system format) which makes using a single pkgtool variable difficult.
Also, SYSV packages in datastream format don't necessarily have uniform
names.  Therefore, use the existing 'simplename' attribute to specify
the datastream file name.
I'm not sure I understand the changes to `Install()` -- it looks like you've just removed the single-pass install?  Can you explain why?